Field
The present invention relates to a light guide plate and a surface illumination device, and more specifically, to a light guide plate and a surface illumination device that spread light emitted from a light source through a surface, and emit the light from a light output surface.
Related Art
Related Art: Example 1
FIG. 1 is a perspective view illustrating a surface illumination device disclosed in Japanese Unexamined Patent Publication No. 2014-146535. In the surface illumination device 11 in FIG. 1, a wedge-shaped light conducting portion 14, and a planar main light guiding body 15 are integrated to form the light guide plate 13. The light input end surface 16 of the light conducting portion 14 faces a light source 12. The upper surface of the light conducting portion 14 is formed as a slanted surface 17. A light output surface is formed on the upper surface of the main light guiding body 15.
The upper surface of the light conducting portion 14, i.e., the slanted surface 17 is provided with a light diffusing pattern 18 that broadens the directivity of the light reflected thereby in the transverse direction. A lenticular lens 20 is formed on the light output surface 19 of the main light guiding body 15. A plurality of V-shaped radial pattern elements, which are directivity conversion patterns 21 is provided at the end portion of the main light guiding body 15, i.e., the region sandwiched between the light output surface 19 and the slanted surface 17, while a non-patterned region 22 is provided in front of the light source 12.
As illustrated by the arrows in FIG. 1, the light from the light source 12 entering the light conducting portion 14 from the light input end surface 16 reflects from the upper and lower surfaces of the light conducting portion 14 and travels toward the thinner main light guiding body 15 in the surface illumination device 11. The light diffusing patterns 18 simultaneously broaden the directivity of the light in the width direction of the light conducting portion 14. The light guided into the main light guiding body 15 reflects from the directivity conversion patterns 21, the lenticular lens 20, and the lower surface of the main light guiding body 15 while travelling through the main light guiding body 15. At this point, the light incident on the directivity conversion patterns 21 reflects from the directivity conversion patterns 21 bent outward in the width direction to spread in the width direction of the light guide plate 13. A portion of the light bent outward due to the directivity conversion patterns 21 reflects from the side surfaces of the light guide plate 13 and returns to the center of the light guide plate 13. In this manner, the lenticular lens 20 widens the directivity of the light spreading throughout the main light guiding body 15 while ensuring the light exits substantially uniformly from the light output surface 19.
However, uneven luminance is more likely to occur in this kind of surface illumination device 11 when the position of the light source 12 shifts in the width direction. FIG. 3A, FIG. 3B, and FIG. 3C depict a simulation of the luminance distribution when the light source 12 is positioned at the center, shifted 0.1 mm from the center, shifted 0.2 mm from the center of the light input end surface 16 in the width direction, respectively. The pitch of the directivity conversion patterns 21 is roughly 0.025 mm in the simulations.
The luminance distribution for FIG. 3A is symmetric about the center of the light source 13 (i.e., the optical axis of the light source). In contrast, the luminance distribution for FIG. 3B is slightly asymmetrical, while the luminance distribution for FIG. 3C is markedly asymmetrical. Therefore, the shifted the position of the light source tends to create less uniformity in the luminance distribution, leading to an uneven luminance in the surface illumination device.
The reason the shifted position of the light source tends to create the above-mentioned kind of asymmetrical luminance distribution is described with reference to FIG. 2A and FIG. 2B. The directivity conversion patterns 21 in the surface illumination device 11 form a radial pattern. Therefore when the position of the light source 12 coincides with the center of the directivity conversion patterns 21, the light reflects symmetrically therefrom as illustrated in FIG. 2A. However, when the position of the light source 12 is shifted from the center of the directivity conversion patterns 21, the directions in which the directivity conversion patterns 21 reflect the light become asymmetrical as illustrated in FIG. 2B, and the directions of the reflected light varies greatly from the directions illustrated in FIG. 2A. Therefore, the guiding direction in the main light guiding body 15 bends as represented by the arrow in FIG. 3C, and the luminance distribution is asymmetrical.
Related Art: Example 2
FIG. 4A and FIG. 4B are perspective views illustrating a portion of a light guide plate disclosed in the “The Design of Efficiency Light-Guide Plate with Multi Steps Wedge Structure”, Yi Wen Chang (Auto Technology Center, AU Optronics Corporation), IDW '13 FM Cp-21, p. 634-636. Two types of patterns 27, 28 are provided on the slanted surface 17 formed on the upper surface of the light conducting portion 14 in the light guide plate 26. One of the patterns 27 is relatively short and extends from the light input end surface 16; the other of the patterns 28 is relatively long and extends from the light input end surface 16. The pattern 27 and the pattern 28 are lined up next to each other, and the inclination of the ridges in the pattern 28 is smaller than the inclination of the ridges in the pattern 27.
FIG. 5 illustrates a luminance distribution in the light output region in a surface illumination device using the light guide plate 26. When a distance D from the light input end surface 16 to the end of the light output surface 19 is small (in the simulation, D=2.5 mm) in a light guide plate 26 constructed as illustrated in FIGS. 4A and 4B, a high luminance region is created at the front surface of the light source 12 at the end portion of the light output surface 19, resulting in a non-uniform luminance distribution as illustrated in FIG. 5.    Patent Document 1: Japanese Unexamined Patent Publication No. 2014-146535    Non-Patent Reference 1: “The Design of Efficiency Light-Guide Plate with Multi Steps Wedge Structure”, Yi Wen Chang (Auto Technology Center, AU Optronics Corporation), IDW '13 FM Cp-21, p. 634-636